Composition for increasing activity of a no-clean flux

ABSTRACT

An activated no-clean flux composition for soldering includes a dicarboxylic acid, an organic solvent, and acetic acid in the range of about 2% to about 4% by weight.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention pertains to a flux composition for soldering, for example,a semiconductor chip or a chip carrier module to a printed circuitboard.

2. Description of the Related Art

Fluxes play an important role in the procedures used to mount electroniccomponents onto printed circuit cards and printed circuit boards (bothof which are hereinafter generically referred to as printed circuitboards or PCBs). For example, one method for directly mounting asemiconductor integrated circuit device (hereinafter called asemiconductor chip or just a chip) onto a PCB is, for example, to formregions of solder, e.g., solder balls, on contact pads on thecircuit-bearing surface of the chip. Such solder regions may also beformed on corresponding contact pads on the PCB. Then, a flux is appliedto the solder regions on the chip and/or to the corresponding contactpads and/or corresponding solder regions on the PCB in order to removeoxide layers which may have formed on these solder regions or contactpads and to achieve increased wetting of the contact pads by the solderregions. Thereafter, with the circuit-bearing surface of the chip facingthe PCB, the solder regions on the chip are brought into contact withthe corresponding contact pads or solder regions on the PCB, and theresulting assembly is heated in order to melt, and thereby reflow, thesolder regions on the chip and/or the PCB. Upon cooling andre-solidification, solder connections between the chip and the PCBresult.

In a manner similar to that described above, one method for mounting amodule, e.g., an organic module or a ceramic module, bearingsemiconductor chips (hereinafter denominated a chip carrier module orjust module) onto a PCB, involves forming, e.g., screening, regions ofsolder onto contact pads on the non-chip-bearing surface of the module.Such solder regions may also be formed on corresponding contact pads onthe PCB. A flux is then applied to the solder regions on the moduleand/or the corresponding contact pads and/or corresponding solderregions on the PCB. Thereafter, with the non-chip-bearing surface of themodule facing the PCB, the solder regions on the module are brought intocontact with the corresponding contact pads or solder regions on the PCBand the resulting assembly is heated in order to melt, and therebyreflow, the solder regions on the chip and/or the PCB. Upon cooling andre-solidification, solder connections between the module and the PCBresult.

If the module of interest has electrically conductive pins extendingfrom the non-chip-bearing surface of the module, then the module ismounted onto a PCB by, for example, initially positioning the moduleover the top (i.e., the circuit-bearing) surface of the printed circuitboard and inserting the electrically conductive pins of the module intocorresponding, copper plated through holes (PTHs) extending through thethickness of the PCB. Then, the PCB and the module are placed on aconveyor, which passes the PCB and module over a fluxing wave or fluxsprayer, which serves to impinge liquid flux onto the bottom surface ofthe PCB and into the PTHs. This flux is wicked up into the PTHs, andthus flux is applied to both the walls of the PTHs and to the pinsextending into the PTHs. Thereafter, the conveyor passes the PCB andmodule over a solder wave, which serves to impinge liquid solder ontothe bottom surface of the PCB and into the PTHs. This liquid solder isalso wicked up into the PTHs, filling the PTHs and, upon cooling andsolidification, serving to encapsulate the pins within the PTHs.

One of the most important aspects of the above-described chip-mountingand module-mounting procedures is the choice of flux. That is, as notedabove, the flux serves to remove any oxide layers which may have formedon the solder regions, contact pads, pins or PTHs and to increase thewetting of, for example, contact pads by solder regions. A problem withcommonly available fluxes is degraded flux components that interferewith underfill adhesion in soldered connections. The underfill adhesionis provided by a liquid adhesive having a low viscosity that is appliedto soldered connections, such as those between chips and chip carriers,to fill in underneath for greater strength. Another problem is that, inmost instances, at the completion of the soldering process, use of thecommonly available fluxes results in ionic residues remaining on thesolder regions, contact pads, pins or PTHs. Such ionic residues areundesirable because they lead to corrosion of circuitry and to shortcircuits. Consequently, if formed, such ionic residues must be removed,e.g., cleaned with water, after the completion of the soldering process.

The solder connections formed between a chip and a PCB or between apinless module and a PCB, as described above, have relatively smallheights, e.g., 4 mils, and therefore the spacing between a chip orpinless module and its PCB is correspondingly small. This is significantbecause it implies that it would be very difficult, if not impossible,to clear away any ionic residues remaining on the solder regions and/orcontact pads after the completion of the soldering process. In addition,in the case of a pinned module, while corresponding ionic residues arereadily cleaned with water, one must then deal with the environmentalhazards posed by the resulting contaminated water.

Significantly, those engaged in the development of fluxes and solderingprocesses for mounting chips and modules onto PCBs have sought no-cleanfluxes, which leave essentially no ionic residues on solder regions,contact pads, pins or PTHs at the completion of the correspondingsoldering processes. As is described in U.S. Pat. No. 5,531,838, oneno-clean flux includes pimelic acid, HOOC(CH₂)₅COOH, as the primaryactive ingredient, and two organic solvents, one with a relatively lowevaporation temperature and one with a relatively high evaporationtemperature.

No-clean fluxes are typically formulated to provide for completevolatility during reflux. As a consequence of this requirement, theactive ingredients of the fluxes usually consist of weakly activevolatile carboxylic acids dissolved in non-active volatile solvents.These fluxes may work very well as long as surface oxide thickness iskept to a minimum. However, when thicker oxides are present, such asthose encountered with electrodeposited solders, significant non-wetsmay be observed. Furthermore, addition to these fluxes of typicalactivators, such as chlorinated or brominated amines or alcohols,results in residues which might not be tolerable.

SUMMARY OF THE INVENTION

By the present invention, a composition is provided which has anactivator which enhances the solder wetting properties of a dicarboxylicacid and is completely volatile in a typical reflow profile. As aresult, non-wets when plated solders are used are reduced to acceptablelevels, while the low residue property of the flux is preserved.

More specifically, a low concentration of acetic acid, in the range ofabout 2% to about 4%, is added to a flux solvent system containing adicarboxylic acid, such as adipic, pimelic or sebacic. Acetic acid, byitself in the solvent system, does not have noticeable fluxingcharacteristics. Glacial acetic acid can serve as a flux, but heavyresidues of lead around the solder sites are usually obtained. Suchresidues can be detrimental to the electrical properties. However, whenacetic acid is added to a flux containing a dicarboxylic acid, it actsas a precursor to the formation of the higher molecular weightcarboxylate lead and tin salts. The result is that a more completereaction is achieved with deeper oxides and, as the temperature israised during the reflux profile, the acetate ion is replaced by thehigher molecular weight species.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The invention involves a flux composition which leaves essentially noionic residues at the completion of conventional soldering processesused to mount electronic components onto PCBs. Consequently, there is noneed to clear away such ionic residues at the completion of thesesoldering processes with, for example, water, and therefore there is noneed to deal with the environmental hazards posed by water contaminatedwith ionic residues.

The invention also involves the application of the flux composition tosoldering processes used to mount electronic components, such as chips,chip carrier modules, resistors, capacitors, etc. onto PCBs.

Although acetic acid by itself does not have noticeable fluxingcharacteristics, when added in a low concentration to a solvent systemcontaining a longer-chain dicarboxylic acid, it results in a morecomplete reaction with deeper oxides and, as the temperature is raisedduring reflow, the acetate ion is replaced by the higher molecularweight species. Usually, the amount of acetic acid in the composition isabout 2% by weight. However, if the carboxylic acid is less than about4.5% by weight, the amount of acetic acid can be increased from 2% toany amount up to 4% by weight.

Significantly, the flux composition includes a carboxylic acid as theactive ingredient, i.e., as the primary fluxing agent. It should benoted that, at room temperature, carboxylic acids such as pimelic,adipic and sebacic are solids, and have respective melting temperaturesof about 152 degrees C., about 105 degrees C., and about 134 degrees C.In addition, the flux composition of the present invention includes twoorganic solvents, the first of which has a relatively low evaporationtemperature, e.g., 82.4 degrees C., and the second of which has arelatively high evaporation temperature, e.g., about 170 degrees C. Theflux composition can also include a relatively small amount of water,preferably de-ionized water. Among carboxylic acids, pimelic acid(HOOC(CH₂)₅COOH) is preferred. It should be noted that pimelic acid, thesecond organic solvent and the water are soluble in the first organicsolvent.

The first organic solvent is preferably isopropanol (isopropyl alcohol),which has an evaporation temperature of 82.4 degrees C. Usefulalternatives to isopropanol include n-propanol and benzyl alcohol.

The second organic solvent is preferably propylene glycol monobutylether (also denominated N-butylpropylglycol ether), which has anevaporation temperature of about 170 degrees C. Useful other examplesare glycol monobutyl ether include propylene glycol monopropyl ether anddiethylene glycol monomethyl ether. Where pimelic acid is used as thecarboxylic acid, upon evaporation of the first organic solvent duringthe soldering process, the pimelic acid (and the water, if present) isthen substantially dissolved in the second organic solvent, until thesecond organic solvent evaporates during the soldering process.

The relative amount of carboxylic acid in the inventive flux compositionranges from about 1% to about 9% by weight. If the inventive fluxcomposition is to be used, for example, in soldering a semiconductorchip to a PCB, then the relative amount of carboxylic acid is preferably4.5% by weight. Relative amounts of carboxylic acid less than about 1%by weight are undesirable because they result in insufficient and/orinadequate fluxing action, i.e., insufficient and/or inadequate removalof oxide layers and insufficient reductions in solder surface tensions.Relative amounts of carboxylic acid greater than about 9% by weight areundesirable because they result in undesirably large amounts of residuesat the completion of conventional soldering processes.

The first organic solvent, e.g., isopropanol, comprises about 75% byweight of the organic solvents overall, whereas the second organicsolvent comprises about 25%.

The relative amount of water, if used, in the inventive flux compositionranges from 0% to about 2% by weight. The purpose of the water, ifpresent, is to enhance the low temperature mobility of positivelycharged ions to accelerate the initiation of fluxing action by thepimelic acid. Relative amounts of water greater than about 2% by weightare undesirable because this significantly increases the possibilitythat the application of the inventive flux composition will result inionic residues.

After the carboxylic acid, the acetic acid and any water are present inthe relative amounts described above, the organic solvent comprises therest of the inventive composition, the organic solvent containing thefirst organic solvent and the second organic solvent in a weight ratioof about 3 to 1.

By way of example, one embodiment of the inventive flux compositionwhich is useful in soldering a semiconductor chip to a PCB is readilyformed by dissolving 4.5 grams of pimelic acid, 2.0 grams of aceticacid, 25.0 grams of propylene glycol monobutyl ether and 1.0 grams ofde-ionized water in 75.0 grams of isopropanol.

In mounting a semiconductor chip, such as in the so-called flip-chipconfiguration, onto a PCB, contact pads on the circuitry-bearing surfaceof the chip are provided with solder regions, e.g., solder balls. Thesesolder regions have compositions which include, for example, 97 atomicpercent lead (Pb) and 3 atomic percent tin (Sn). Significantly, suchsolder regions have relatively high melting temperatures, and do notmelt during the soldering process described below.

Prior to soldering the chip to the PCB, contact pads on thecircuitry-bearing surface of the PCB are provided with relatively smallsolder regions, e.g., relatively small solder balls. These relativelysmall solder regions are readily transported to, and deposited on, thecontact pads via a decal. By contrast with the solder regions used withthe chip, the solder regions of the PCB have compositions which include,for example, 37 atomic percent Pb and 63 atomic percent Sn. These solderregions have melting temperatures of 183 degrees C. and do melt duringthe soldering process described below.

Prior to soldering the chip to the PCB, the inventive flux compositionis applied to the solder regions of the chip, and/or the contact pads onthe PCB, and/or the contact pads on the chip. This is readilyaccomplished using, for example, a syringe or a brush.

Having applied the inventive flux composition to the relevant solderregions and/or contact pads, the chip is positioned relative to the PCBso that the solder regions of the chip contact the solder regions of thePCB. Consequently, these combined solder regions substantially extendfrom the chip contact pads to the PCB contact pads.

With the solder regions of the chip and the PCB touching each other, thechip PCB assembly is heated in, for example, an oven. During thisheating procedure, the oven temperature is initially raised to about 183degrees C., and subsequently raised to about 250 degrees C. Then, theoven temperature is lowered to about 183 degrees C., and thereafterlowered to room temperature. As a consequence, the solder regions of thePCB undergo melting and flow around the solder regions of the chip,resulting in continuous metallurgical and electrical connections betweenthe PCB and the chip. While the cleaning of these continuous connectionswould be extremely difficult, and perhaps even impossible, no suchcleaning is needed because essentially no ionic residues remain at thecompletion of this soldering process.

At the completion of the above-described soldering process, thecontinuous solder connections between the PCB and the chip arepreferably encapsulated in, for example, an epoxy resin, usingconventional techniques.

If the electronic component to be mounted onto a PCB is, for example, a(pinless) chip carrier module bearing at least one semiconductor chip,then such a module is readily mounted by, for example, screening solderregions onto contact pads on the non-chip-bearing surface of the module.Such solder regions may also be screened onto corresponding contact padson the PCB. The inventive flux composition is then applied to the solderregions and/or the module contact pads and/or the PCB contact pads,using, for example, a syringe or a brush. Thereafter, the module ispositioned in relation to the PCB so that the solder regions on themodule contact pads touch the solder regions on the PCB contact pads.Thus, these combined solder regions substantially extend from the modulecontact pads to the PCB contact pads. Then, with the module solderregions touching the PCB solder regions, the module/PCB assembly isheated in, for example, an oven in order to melt the module solderregions and/or the PCB solder regions.

By contrast with the above, if the electronic component to be mountedonto a PCB is, for example, a pinned chip carrier module bearing atleast one semiconductor chip, then such a module is readily mounted byinitially applying the inventive flux composition to the module pinsand/or to the walls of corresponding PTHs in the PCB. This is readilyaccomplished (using any of a variety of conventional techniques) beforethe module pins are inserted into the PTHs, while the module pins arebeing inserted into the PTHs, or after the module pins are inserted intothe PTHs. Preferably, this is accomplished after the module pins havebeen inserted into the PTHs by, for example, placing the module/PCBassembly on a conveyor which passes this assembly over a fluxing wave ora flux sprayer. This fluxing wave or flux sprayer serves to impinge theinventive flux composition onto the bottom surface of the PCB and intothe PTHs. The impinged flux is wicked up into the PTHs, and thus theinventive flux composition is applied both to the walls of the PTHs andto the module pins. Thereafter, the conveyor preferably serves to passthe module/PCB assembly over a solder wave, which serves to impingeliquid solder onto the bottom surface of the PCB and into the PTHs. Thisliquid solder is also wicked up into the PTHs, filling the PTHs and,upon cooling and solidification, serving to encapsulate the pins withinthe PTHs.

If the electronic component to be mounted onto a PCB is, for example, adiscrete, passive electronic component, such as an electrical resistoror capacitor, having leads instead of pins, then such an electroniccomponent is readily mounted using a procedure which is almost the sameas the used with a pinned chip carrier module. The only difference isthat the leads of the discrete, passive electronic component are notpositioned inside the PTHs. Rather, these leads are positioned adjacentthe PTHs, e.g., these leads are placed in contact with the landsencircling the PTHs. Thus, when the component/PCB assembly is passedover the fluxing wave or flux sprayer, the inventive flux composition iswicked up into the PTHs, onto the lands and onto the bottom portions ofthe leads. Similarly, when the component/PCB assembly is passed over thesolder wave, liquid solder is wicked up into the PTHs, onto the landsencircling the PTHs and onto the bottom portions of the leads.

While the invention has been particularly described with reference topreferred embodiments thereof, it will be understood by those skilled inthe art that various changes in form and details may be made thereinwithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A no-clean flux composition for use in soldering,consisting essentially of: a dicarboxylic acid; an organic solvent; andacetic acid in a concentration in the range of from about 2% to about 4%by weight.
 2. The no-clean flux composition of claim 1, wherein thedicarboxylic acid is selected from the group consisting of adipic acid,pimelic acid, sebacic acid and combinations thereof.
 3. The no-cleanflux composition of claim 2, wherein the dicarboxylic is pimelic acid.4. The no-clean flux composition of claim 1, wherein the dicarboxylicacid comprises about 1% to about 9% by weight of the composition.
 5. Theno-clean flux composition of claim 4, wherein the dicarboxylic acid ispimelic acid.
 6. A no-clean flux composition comprising: a dicarboxylicacid; an organic solvent; and acetic acid, wherein the dicarboxylic acidcomprises about 4.5% by weight of the composition, and the acetic acidcomprises about 2% by weight of the composition.
 7. The no-clean fluxcomposition of claim 6, wherein the dicarboxylic acid is pimelic acid.8. A no-clean flux composition comprising: a dicarboxylic acid; anorganic solvent; and acetic acid in a concentration in a range of fromabout 2% to about 4% by weight, wherein the organic solvent comprises afirst organic solvent selected from the group consisting of isopropanol,n-propanol and benzyl alcohol, and a second organic solvent selectedfrom the group consisting of propylene glycol monobutyl ether, propyleneglycol monopropyl ether, and diethylene glycol monomethyl ether.
 9. Theno-clean flux composition of claim 8, wherein the first organic solventcomprises about 75% by weight of the organic solvent and the secondorganic solvent comprises about 25% by weight of the organic solvent.10. The no-clean flux composition of claim 8, wherein the first organicsolvent is isopropanol, and the second organic solvent is propyleneglycol monobutyl ether.
 11. The no-clean flux composition of claim 6,further comprising water in the amount of 0% to about 2% by weight. 12.The no-clean flux composition of claim 8, wherein the dicarboxylic acidis selected from the group consisting of adipic acid, pimelic acid,sebacic acid and combinations thereof.
 13. The no-clean flux compositionof claim 12, wherein the dicarboxylic is pimelic acid.
 14. The no-cleanflux composition of claim 12, wherein the dicarboxylic acid comprisesabout 1% to about 9% by weight of the composition.
 15. The no-clean fluxcomposition of claim 6, wherein the dicarboxylic acid is selected fromthe group consisting of adipic acid, pimelic acid, sebacic acid andcombinations thereof.